(1) Field of the Invention
The present invention relates to a resin composition applied to electric insulation materials for vehicles. More specifically, it relates to a resin composition for distributor caps which is prepared by blending a crystalline propylene-ethylene copolymer resin with glass fibers, talc or mica, silica, modified polyolefin resins, halogen-containing flame retardants and flame retardant auxiliaries.
(2) Description of the Prior Art
Heretofore, as materials for distributor caps have been used, for example, an epoxy resin, a polybutylene terephthalate resin containing glass fibers ( hereafter referred to as the glass fiber-containing PBT), a polypropylene resin containing talc and a polypropylene resin containing talc and flame retardants.
The electric insulation materials for vehicles in this invention mean the electric insulation materials for motor vehicles, used at high temperature, under often wet contaminated circumstances, like in an engine room or compartment, and at high voltage. Such material is typically used for distributor caps.
The epoxy resins have a long history and have been used extensively in many fields, but their product cost is high because of a high material cost and manufacturing cost, and they are poor in impact resistance and, therefore, are liable to break during transportation disadvantageously. The glass fiber-containing PBT and the talc-containing polypropylene resin can solve the above problems. However, glass fiber-containing PBT has a poor tracking resistance, and, therefore, when distributor caps comprising the glass fiber-containing PBT are used in a briny air environment, the outer surfaces of the distributor caps are partially carbonized, so that the problem of bad insulation occurs. As for the talc-containing polypropylene resin, mechanical characteristics (stiffness) at high temperatures weld characteristics and the ability to withstand voltage duration (hereinafter referred to corona resistance) are poor. Furthermore, a common problem of glass fiber-containing PBT and talc-containing polypropylene is that flame retardance is poor.
The reason why the tracking resistance is necessary is as follows: When the distributor caps are under circumstances where their surfaces are liable to be brought into contact with a saline solution or the like, as in a briny air environment,the surfaces are contaminated with moisture and further partially carbonized by a minute electric discharge so as to allow an electrical conduction therethrough, so that their insulation properties become bad.
The tracking resistance as surface characteristics is due to the molecular structure of the resin material. A resin such as the PBT which has a benzene ring in the molecule is bad in tracking resistance. This reason is as follows: Carbon bonds in the benzene ring are firm, and therefore the resin is hard to gasify by a heat energy of the minute discharge, so that conductive free carbon having a graphite structure, is apt to be left on the surfaces. To the contrary, polypropylene resin has no benzene ring in the molecule, and therefore its tracking resistance is extremely good. Accordingly, from the viewpoint of the tracking resistance, the polypropylene resin is most desirable as a material for distrbutor caps.
The reason why the above-mentioned mechanical characteristics (stiffness) at high temperature is necessary is as follows: In recent years, conditions in the vicinity of an automotive engine have been severe and, with regard to the distributor cap under such conditions, a higher heat resistance is required. However, the talc-containing polypropylene resin has poor mechanical characteristics (stiffness) at high temperature ( e.g., such temperature as to cause a thermal distortion under a fiber stress of 18.6 kg/cm2 : usually 90.degree. to 100.degree. C.). Therefore, when a tower portion of cap onto which a load from a high-tension cord is applied is vibrated at a high temperature, this portion tends to be deformed and damaged.
The above-mentioned problems of the weld characteristics include a problem of bad weld appearance which looks as if cracks have occurred in the molded distributor caps, and another serious problem that cracks occur in electrode portions of the caps and a weld portion on the periphery of insert fitments. That is, such cracks in the weld portion are caused by a progressive crystallization subsequent to molding, by post shrinkage and by heat stress generated from a cooling/heating cycle when the caps are exposed to high-temperature conditions in an engine room of an automobile for a long period of time, or when exposed to rapid temperature change conditions in winter.
The corona resistance can be defined as follows: When the surfaces of distributor caps are contaminated with a salline solution or the like as in a briny air district, and when they are exposed to corona discharge for a long period of time, insulating properties of the distributor caps deteriorate in a vertical direction thereof, and finally dielectric breakdown occurs in the distributor caps. Therefore, the corona resistance can be defined as properties which withstand voltage duration of the above-mentioned corona discharge in the vertical direction.
Further, the reason why the above-mentioned flame retardance is necessary is as follows: When a connector for distribution is fixed improperly, an arc discharge is generated during driving an automobile and the tower portion of a distributor cap is carbonized by a discharge energy of this arc discharge. When the carbonization progresses, the cap must be replaced with a new one.
Polypropylene resin containing talc and a flame retardance can eliminate the drawback of poor flame retardance which glass fiber-containing PBT and talc-containing polypropylene resins have. With regard to the polypropylene resin containing talc and flame retardant, however, as a result of the incorporation of the flame retardant, the mechanical characteristics (tensile strength) decline, and in addition, the poor weld characteristics which are the drawback of the talc-containing polypropylene resin are worsened. Furthermore, when this resin is used under high-temperature conditions, the used flame retardant blooms to impair its appearance and flame retardance decline disadvantageously.
In view of the above-mentioned conventional technical problems, the object of the present invention is to provide a resin composition suitable for electric insulation materials for vehicles in which balance is kept in all points of cost, workability, impact strength, mechanical characteristics (tensile strength), mechanical characteristics (stiffness) at high temperature, tracking resistance, weld characteristics, corona resistance and flame retardance.